So calling at 8am worked. I dialled 0344 493 0787 (the number shown if you try and alter your flights online) at 7.58:30. Sat through the 90+second blurb and after option selection the call was answered within a few seconds.
The irony is that I've ended up having to cancel and get a voucher for the £150 deposit in which case you have to do this online.